1. Field of the Invention
The present invention relates to a backlight LED drive circuit for driving LED (Light Emitting Diode) used for a backlight of a liquid crystal panel.
2. Description of the Related Art
A backlight LED drive circuit for driving LED used for a backlight of a liquid crystal panel is well known. FIG. 5 is a block diagram showing a conventional backlight LED drive circuit. Referring to FIG. 5, a backlight LED drive circuit 1 includes a step-up DC/DC converter 2, voltage detector 3, and PWM control circuit 4. The drive circuit 1 steps up a DC supply voltage based on a PWM signal output from the PWM control circuit 4 and supplies the stepped up voltage to an LED device 5 formed of a plurality of LEDs connected in series. At this time, a feedback voltage (FB voltage) based on the voltage on the cathode side of the LED device 5 is detected and the PWM signal is controlled so that the feedback voltage may become a predetermined voltage. For example, when the feedback voltage becomes higher than the predetermined voltage, the duty ratio of the PWM signal is decreased so that the stepped up voltage is lowered, and when the feedback voltage becomes lower than the predetermined voltage, the duty ratio of the PWM signal is increased so that the stepped up voltage is raised.
There are proposed various drive circuits for driving an LED device suitably. For example, there is disclosed, in JP-A 2007-96296 (hereinafter called Patent Document 1), a display apparatus capable of preventing continuous increase in its standby power when a light source is turned off, thereby improving its display characteristic. In JP-A 2007-13183 (hereinafter called Patent Document 2), there is disclosed a backlight LED drive circuit having a PWM control unit for ON/OFF controlling a switch with a switching pulse having a duty ratio determined in accordance with a predetermined internal reference voltage and a detected voltage detected by a voltage detecting resistor. Further, in JP-A H03-255684 (hereinafter called Patent Document 3), there is disclosed a drive circuit of a light emitting device having its oscillator circuit provided with a switch as a control means for controlling oscillation to be made and stopped, and thereby deterioration or breakage of the light emitting device is prevented and the current flowing through the light emitting device is switched at a high speed.
Now, in the backlight LED drive circuit 1 shown in FIG. 5, if for example the LED device 5 becomes unconnected with the connection terminals 2a, 2b or the LED device 5 becomes broken so as to render the connection terminals 2a, 2b open, then a voltage will not be applied to the connection terminal 2b. Hence the feedback voltage will become lower than the predetermined voltage to increase the duty ratio of the PWM signal, and thereby the stepped up voltage output from the connection terminal 2a may be increased. At this time, even if the stepped up voltage rises, the feedback voltage remains lower than the predetermined voltage. Therefore, there arises a possibility for example that the duty ratio of the PWM signal is kept at its maximum value and the stepped up voltage at the connection terminal 2a continues to rise until it reaches the maximum voltage that the step-up DC/DC converter 2 can output.
The circuit elements constituting a step-up DC/DC converter 2, in general, are designed to have some margin to sustain normal operations. However, in view of the balance between the margin and such a factor as an increase in cost and installation space, it is considered desirable not to use such circuit elements that can stand the above mentioned maximum voltage. Hence, when the connection terminals 2a, 2b have been rendered open during the operation of the backlight LED drive circuit 1, there has been a possibility of troubles occurring in the circuit elements making up the step-up DC/DC converter 2.
Further, when the LED device 5 has been connected between the connection terminals 2a, 2b under the condition where the stepped up voltage at the connection terminal 2a is high, there has been a possibility of troubles occurring in the LED device 5.
While the arts disclosed in the above mentioned patent documents are not such that prevent occurrence of those troubles, there has not yet been proposed any art to prevent occurrence of troubles when the connection terminals 2a, 2b are rendered open.